The present invention relates generally to methods of and systems for capturing fingerprint images, and more particularly to a semiconductor capacitive fingerprint scanning device.
Fingerprint recognition has been suggested for use in many security applications such as controlling access to buildings, computers, or the like. Fingerprint recognition systems enable a user to access the controlled facility without a device such as a key or smart card or without having to memorize a password or other personal identification number.
The sensing device is an important part of a fingerprint recognition system and the quality of the representation of the fingerprint that the device produces will affect recognition capability and the amount of processing required for verification of the fingerprint. Various technologies have been proposed for use in fingerprint sensing devices. One commonly proposed technology involves optical detection. Examples of optical fingerprint detection devices are disclosed in Jensen, U.S. Pat. No. 4,784,484; Fishbine, et al., U.S. Pat. No. 5,467,403; and Giles, et al., U.S. Pat. No. 5,548,394.
Optical detectors include a glass surface upon which a subject places his finger to be recognized. Optical detectors may present recognition problems when the glass surface or the subject""s finger is wet. The optics of the detectors are constructed based upon the indices of refraction of air and glass. When water or perspiration is between the glass and the surface of the finger, the operation of the detector is affected.
In addition to optical sensors, various electrical sensor systems have been proposed, as for example in Knapp, U.S. Pat. No. 5,325,442; Tamori, U.S. Pat. No. 5,400,662; and Tamori, U.S. Pat. No. 5,429,006. The electrical detection devices typically comprise an array of sense elements. The individual sense elements respond with an output that depends upon whether a fingerprint ridge or valley is located over the sense element.
The electrical detection devices offer advantages over the optical detection device. However, an electrical detector that is large enough to detect a fingerprint is a large and expensive semiconductor device. For example, the TouchChip(trademark) Silicon Fingerprint Sensor (STFP2015-50) available from STMicroelectronics, Inc. has an active sensor surface measuring 19.2 mm by 12.8 mm that includes a 384 by 256 sensor array. Accordingly, electrical detection devices tend to be more expensive than optical detectors.
It is an object of the present invention to provide a fingerprint detecting device that overcomes the shortcomings of the prior art.
The present invention provides a scanning fingerprint detection system that includes an array of capacitive sensing elements lying beneath the top surface of a protective layer. The array has a first dimension about the width of a fingerprint and a second dimension substantially less than the length of a fingerprint. Each of the capacitive sensing elements has first and second thin conductor plates, which are closely spaced relative to each other, and an inverting amplifier having an input connected to the first conductor plate and an output connected to the second conductor plate, the amplifier generating a signal indicative of a ridge or a valley of a fingerprint of a finger pressed against the top surface of the protective layer. Circuitry is provided for scanning the array to capture an image of a portion of fingerprint and for assembling the captured images into a fingerprint image as a fingerprint is moved over the array parallel to the second dimension.